The invention relates to the system disclosed in the U.S. Pat. No. 3,872,451 issued to this inventor Mar. 18, 1975, and utilizes therein certain teachings set forth in U.S. Pat. No. 3,804,489 concerning induced stationary diffraction gratings. Such gratings are produced by an interdigital electrode structure which is deposited on an electro-optic material. The application of a voltage to the structure produces a spatial modulation of the electro-optic material's refractive index and thereupon allowing the material to act as a diffraction grating to light incident thereon.